July/August 2021 - 27

30
25
20
15
10
5
Alturas
1.090
1.088
1.086
1.084
1.082
1.080
1.078
40
60
80
100
120
Late season irrigation level (% evapotranspiration)
Figure 2
As irrigation level declined, the percent
of total yield comprised of U.S. No. 1
grade tubers increased; higher irrigation
levels essentially led to larger, uglier
tubers. As an example, Alturas plants
produced more growth crack afflicted
tubers as irrigation levels increased
(Figure 2). Close to 25% of Alturas tubers
contained growth cracks when irrigation
was excessive (120% ET).
Another factor affecting gross return
for all cultivars was a loss in specific
gravity as irrigation levels increased
(Figure 3). Plants receiving less irrigation,
produced tubers with more dry matter
than those under higher irrigation levels.
French fry color for all cultivars was
reduced as irrigation levels increased
during the last half of the season;
however, not all cultivars experienced
the same loss in fry color, nor was it
always significant to market acceptance.
CONCLUSIONS
Avoiding excessive irrigation during
the last half of the season will improve
tuber quality and reduce growth cracks
(Alturas). All cultivars produced the best
economic return and yield when cultivars
were irrigated at or above 80% ET from
mid-season to vine kill. We recommend
Alturas growers drop from 100% ET to
80% ET during the last four to six weeks
of growth leading up to vine kill. This
should lead to optimum grower return
by maintaining yield and specific gravity
while minimizing tuber growth cracking.
For Clearwater Russet, Ranger Russet
and Russet Burbank, we recommend
mid to late-season irrigation of 100%
ET, and Umatilla 80-100% ET, until
Spudman * July/August 2021
27
Figure 3
about three weeks prior to vine kill/
predicted plant death. Three weeks
prior to harvest, we recommend
growers cut back to 80% ET to allow
plants and tubers to mature naturally.
Reducing late season irrigation may
also lead to improved tuber storability
due to a reduction in diseases and
associated rot, which may develop as
tubers sit in moist or wet soils awaiting
harvest. Moreover, the sudden decrease
in available soil water late in the season is
critical to maximize maturation-induced
effects. Although not exact, 100% ET is
often equivalent to a soil water content
with a range 80-100% of field capacity
during active irrigation and plant
growth. Available soil water at 80% ET is
approximately 65-70% of field capacity.
The authors thank the Northwest
Potato Research Consortium, Washington
State Potato Commission and industry,
and Washington State University for
funding this project. Francisco Gonzalez
conducted this research in pursuit of his
Ph.D. under direction of Mark Pavek,
Washington State University Potato
Research Group.
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Values averaged across 5 cultivars and 3 years
40
60
80
100
Late season irrigation level (% evapotranspiration)
120
Tuber growth cracks (% of total yield)
Tuber specific gravity
July/August 2021

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